Method for logging behind casing

Abstract

A method for logging a subterranean formation from within a well that has been cased and cemented is disclosed. An acoustic signal is transmitted into the formation from a pulse-echo transducer placed within the wellbore and the first casing echo and the first formation echo are detected. The amplitude and the arrival times of these two echoes, combined with some known information about the wellbore, the casing, and the cement enable several parameters to be calculated. These parameters in turn may be used to image the formation around the well.

I claim: 
     
       1. A method for logging a subterranean formation from a wellbore with casing and fill material penetrating said formation, comprising: transmitting an acoustic pulse signal from a pulse-echo transducer into the wellbore at an angle approximately perpendicular to the wellbore;   receiving at the transducer a first casing echo at an angle approximately perpendicular to the wellbore;   receiving at the transducer a first formation echo at an angle approximately perpendicular to the wellbore;   detecting the amplitudes and times of arrival of the casing and formation echoes;   calculating the casing inner radius;   calculating the casing thickness;   calculating the fill material thickness;   calculating the formation reflectance amplitude; and   using the formation reflectance amplitude to image the subterranean formation.   
     
     
       2. The method as defined in claim 1 wherein said step of calculating said casing thickness includes calculating a resonant frequency of said casing. 
     
     
       3. The method as defined in claim 2 wherein said step of calculating said fill material thickness includes determining an acoustic travel time through said casing and through said fill material, said acoustic travel time through said fill material being related to a difference in arrival time between said first casing echo and said first formation echo and to an acoustic travel time through said casing. 
     
     
       4. A method of imaging a subterranean formation using acoustic pulse-echo signals from an acoustic pulse-echo imaging instrument, said formation penetrated by a wellbore, said wellbore including a casing and cement therein, the method comprising: determining amplitudes and first arrival times of echoes from said casing and said formation within said acoustic pulse-echo signals;   determining an inner radius of said casing;   determining a thickness of said casing;   determining a thickness of said cement;   determining a reflectance amplitude of said formation; and   generating an image of said subterranean formation from said reflectance amplitude.   
     
     
       5. The method as defined in claim 4 wherein said step of determining said casing thickness includes calculating a resonant frequency of said casing. 
     
     
       6. The method as defined in claim 5 wherein said step of determining said thickness of said cement includes determining an acoustic travel time through said casing and through said cement, said acoustic travel time through said cement being related to a difference in said first arrival times between said casing echo and said formation echo and to an acoustic travel time through said casing.